1. Field of the Invention
This invention relates to a phase change optical recording medium and method for making such an optical recording medium.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical overwritable optical recording medium is phase change optical recording medium wherein a laser beam is directed to the recording layer to change its crystalline state whereupon a change in reflectance by the crystallographic change is detected for reading of the information. The phase change optical recording media are of great interest since the medium can be overwritten by modulating the intensity of a single laser beam and the optical system of the drive unit is simple as compared to magnetooptical recording media.
Most optical recording media of the phase change type used Gexe2x80x94Te systems which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. It was recently proposed to use new compounds known as chalcopyrites.
Chalcopyrite compounds were investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb2 or IIb-IVb-Vb2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228.
Among the chalcopyrite compounds, AgInTe2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai (JP-A) No. 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992.
In addition to these phase change type optical recording media using chalcopyrite compounds, JP-A 267192/1992, 232779/1992, and 166268/1994 disclose phase change type optical recording media wherein a recording layer crystallizes to create an AgSbTe2 phase.
In the case of prior art phase change type optical recording media, recording layers are formed using vacuum deposition equipment and the as-deposited recording layers remain amorphous with low reflectance. The recording layers must be crystallized by an operation generally known as initialization before the recording media can be utilized as rewritable media.
Initialization is carried out in various ways, for example, after a recording layer is formed on a substrate, by heating the substrate to the crystallization temperature of the recording layer for crystallization as disclosed in JP-A 3131/1990; illuminating a laser beam to the recording layer for crystallization, which method is called solid phase initialization, as disclosed in JP-A 366424/1992, 201734/1990 and 76027/1991; illuminating flash light to the substrate to achieve pseudo-crystallization by so-called photo-darkening, which method takes advantage of the photo characteristics of chalcogen compounds, as disclosed in JP-A 281219/1992; and high-frequency induction heating the medium. JP-A98847/1990 proposes to heat a substrate during formation of a recording layer to thereby crystallize the recording layer. JP-A 5246/1990 discloses a method involving the steps of forming a first dielectric layer, forming a recording layer thereon, heating it for crystallization, and forming a second dielectric layer thereon.
However, the initialization step by laser beam illumination takes a long time and is makes the productivity lower. Heating of the overall medium rejects the use of inexpensive resin substrates. That is, resin substrates can be distorted upon heating for initialization, causing errors in tracking. The method of illuminating flash light is also low in productivity because several shots of illumination are necessary to achieve full crystallization.
Under the circumstances, the use of a so-called bulk erasing is the only technique which is regarded commercially acceptable and currently used. The bulk eraser illuminates a beam from a high power gas or semiconductor laser through a relatively large aperture stop for crystallizing a multiplicity of tracks altogether. Since the bulk eraser permits the recording layer to be locally heated, the substrate temperature is elevated to a little extent, enabling the use of less heat resistant resins as substrates.
Initialization of an optical recording disc with a bulk eraser, however, is a time-consuming process, and it takes several minutes just to initialize the optical recording discs of 12 cm diameter. The process of initialization has been the rate-determining step in the production of the optical recording discs. Elimination or speedup of the initialization step is required for improving the production efficiency.
For the speedup of the initialization process, it is effective to decrease the crystallization temperature of the recording layer. When the recording layer has a lower crystallization temperature, the recording layer will be crystallized even if the bulk eraser was operated at a faster rate. Furthermore, if the crystallization temperature of the recording layer could be reduced to the distortion temperature of polycarbonate, polyolefin and other resins commonly used for the substrate of the optical recording medium about 120xc2x0 C., the initialization can be accomplished simply by heating the medium in an oven and there will be no need to use of the expensive bulk eraser. A significant reduction in the initialization cost by the use of simple initialization process is thereby enabled. At the moment, initialization of the disc in an oven with no distortion in the resin substrate is impossible because the crystallization temperature of the phase change recording materials commonly used in the art, for example, Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te-based materials and Gexe2x80x94Sbxe2x80x94Te-based materials are in the range of about 170 to about 200xc2x0 C.
Various processes are proposed for the purpose of reducing the crystallization temperature of the phase change recording layer. For example, JP-A 106647/1996 proposes a Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te-based recording layer of the structure wherein AgSbTe2 layer and Inxe2x80x94Sb layer, or AgSbTe2 layer, In layer, and Sb layer are separately disposed. In JP-A 106647/1996, the reduced energy for the initialization due to the adoption of the crystallized AgSbTe2 layer is described as its merit. However, composition of the unit layers is limited in JP-A 106647/1996 since it is an object of the JP-A 106647/1996 to form a recording in the form of an artificial lattice film. As a consequence, formation of a recording layer having overall composition optimal for phase change recording is difficult. In addition, since the unit layers in the recording layer are formed as separate layers, formation of a uniform recording layer is difficult even when the recording layer is initialized by heating, and the medium suffer from insufficiently stable properties. It should be noted that the JP-A 106647/1996 is silent about the specific condition used in the initialization (linear velocity, laser power, etc.).
U.S. Pat. No. 4,889,746 proposes lamination of a single element layer such as Sb layer and a low-melting intermetallic compound layer on a heated substrate. In U.S. Pat. No. 4,889,746, composition of the layers is determined such that average composition of these layers fall within the composition of a recording layer. Also disclosed is the capability of reducing the crystallization temperature of each layer. The medium of U.S. Pat. No. 4,889,746 still suffers from insufficient uniformity of the recording layer due to the formation of the unit layers as separate layers, and therefore, from the insufficiently stable properties.
The assignee of the present invention has made proposals in JP-A 221814/1996 and JP-A 226173/1998 to dispense with or speed up the initialization.
JP-A 221814/1996 proposes formation of an Inxe2x80x94Agxe2x80x94Texe2x80x94Sb-based layer by separate steps of (Sb+In) sputtering step and (Ag+Te) sputtering step, or by separate steps of Sb sputtering step, In sputtering step, and (Ag+Te) sputtering step. The recording layer formed by such process is crystallized at least in a part. The recording layer formed by such process exhibits a reflectance equivalent to the one measured after the initialization with a bulk eraser if the medium is repeatedly overwritten and the elements in the recording layer has sufficiently diffused to become mixed with other elements. However, the recording layer formed by such process exhibits unstable degree of erasure as in the case of prior art phase change medium in the overwriting operations from the as-deposited state to several overwriting operations. To be more specific, the reflectance does not become stable until the entire surface is covered by the overwritten region because the reflectance of the region that has been crystallized in the formation of the recording layer is different from the region crystallized in the course of the overwriting operations. In the case of mark edge recording as adopted in rewritable digital video disc (DVD-RAM) and the like, such variation in the reflectance invites erroneous recognition of the mark edge.
JP-A 226173/1998 proposes formation of a recording layer by depositing an Sb-based thin film containing Sb as its main component and a reactive thin film containing In, Ag, and Te, or In, Ag, Te, and Sb as its main components followed by a heat treatment (referred in JP-A 226173/1998 as the xe2x80x9cmixing treatmentxe2x80x9d) for the purpose of mixing these layers. This heat treatment corresponds to the initialization in conventional phase change recording medium in the sense that the as-deposited recording layer is converted to the ready-to-be written state. In the Examples of JP-A 226173/1998, the medium exhibits stable degree of eraser from the first overwriting operation. It has been, however, found in the further investigation that such medium suffers from inconsistent jitter in the first several overwriting operations.
In addition to the publications as described above, JP-A 79144/1998 proposes promotion of crystallization by varying the current density in the course of the recording layer formation by sputtering. The current density used in this proposal, however, is considerably high, and such current density can not be used in the mass production in view of the risk of target destruction.
JP-A 62259/1993 proposes optical heating in the course of the recording layer formation. In this process, however, provision of a high-intensity halogen lamp in the vacuum chamber is required, and crystallization of the recording layer with no resin substrate deformation is impossible.
In view of the situation as described above, an object of the present invention is to reduce the production cost by reducing the time required for the initialization and simplifying the initialization process in the phase change optical recording medium wherein initialization process of the recording layer has been the rate determining and cost-increasing factor in the production of the medium. Another object of the invention is to realize stable writing/reading properties from immediately after the initialization.
Such objects are attained by the present invention as described in (1) to (14), below.
(1) An optical recording medium comprising a substrate and a recording layer formed over the substrate, wherein
the recording layer comprises two or more unit recording layers wherein adjacent two unit recording layers are constituted from different materials and mixture layers between the two adjacent unit recording layers containing all of the elements included in the adjacent two unit recording layers;
at least one of the two adjacent unit recording layers is a crystalline layer; and
the overall composition of the recording layer falls within the composition of a phase change recording material.
(2) An optical recording medium according to the above (1) wherein at least one type of the unit recording layers is formed as a crystalline layer and the mixture layer adjacent to the crystalline unit recording layer is formed as an amorphous layer.
(3) An optical recording medium according to the-above (1) wherein the recording layer after crystallization by heating has an average crystal grain size of 3 to 18 nm in the non-recorded region.
(4) An optical recording medium according to the above (1) wherein said recording layer contains Ag, In, Sb, and Te as the main components.
(5) An optical recording medium according to the above (1) wherein said recording layer contains Ge, Sb, and Te as the main components.
(6) A method for producing an optical recording medium comprising a substrate and a recording layer formed over the substrate by sputtering two or more types of targets, wherein
when two of said targets are designated a first target and a second target, a step is provided wherein the first target and the second target are simultaneously sputtered between the step of sputtering the first target alone and the step of sputtering the second target alone, and
composition of the targets and power supplied are controlled such that the overall composition of the recording layer formed falls within the composition of a phase change recording material.
(7) A method for producing an optical recording medium according to the above (6) wherein the recording layer is subjected to an initialization wherein the recording layer formed is heated for crystallization.
(8) A method for producing an optical recording medium according to the above (6) by which the optical recording medium of the above (1) is produced.
(9) A method for producing an optical recording medium comprising a substrate and a recording layer formed over the substrate by sputtering two or more types of targets, wherein
when three of said targets are designated a first target, a second target, and a third target, the second target contains all of the elements included in the first and the third targets, and the recording layer is formed by the steps wherein the first target, the second target, and the third target are respectively sputtered in this order, and
composition of the targets and power supplied are controlled such that the overall composition of the recording layer formed falls within the composition of a phase change recording material.
(10) A method for producing an optical recording medium according to the above (9) wherein the recording layer is subjected to an initialization wherein the recording layer formed is heated for crystallization.
(11) A method for producing an optical recording medium according to the above (9) by which the optical recording medium of the above (1) is produced.
(12) A method for producing an optical recording medium comprising a substrate and a recording layer formed over the substrate by sputtering two or more types of targets, wherein
the recording layer is formed by a step wherein said substrate is moved such that the substrate is sequentially opposed to said targets while the targets are simultaneously sputtered, and
composition of the targets and power supplied are controlled such that the overall composition of the recording layer formed falls within the composition of a phase change recording material.
(13) A method for producing an optical recording medium according to the above (12) wherein the recording layer is subjected to an initialization wherein the recording layer formed is heated for crystallization.
(14) A method for producing an optical recording medium according to the above (12) by which the optical recording medium of the above (1) is produced.